Ultrasonic and pneumatic lithotripsy or so-called stone-breaking devices have been available for medical use for a number of decades. Currently, there exist a number of rigid solid tubular shaft-based lithotripsy devices that use ultrasonic or pneumatic energy to break a respective stone down into smaller pieces for easier removal from a respective patient's urologic system.
In general, during use of a shaft-based lithotripsy device, ultrasonic acoustic frequency energy is transmitted (translated) down a stiff metal shaft and delivered by contact to a kidney stone. The tips of tubes or shafts in such devices are typically terminated with a flat surface. For procedures performed with the tubular shaft device, liquid and debris can be sucked through the center of the tubular shaft.
Some devices incorporate and deliver a lower frequency energy component to the kidney stone either through the same shaft or via a second shaft; the second shaft is usually coaxial to an ultrasonic energy shaft. Presence of the additional secondary, lower frequency shaft shows evidence of improving the stone breaking efficiency in comparison to an approach in which only a single ultrasonic energy and corresponding shaft is used to break up a kidney stone.
Typically, the use of such a lithotripsy device requires that the stone being broken is pressed up against some surface, usually an inner wall of the kidney, in order that the vibrational energy from the tip of the tool can be sufficiently delivered to the stone surface to break it up. Some devices in the market offer a combination of a lithotripsy shaft and a stone basket where the lithotripsy shaft is incorporated into the center of the lithotripsy basket; the shaft and emerges into the center of the lithotripsy basket. This design offers the ability to apply the pneumatically driven shaft to a kidney stone contained in the associated basket, or if the kidney stone is too large, to extend the shaft beyond the basket to break up a stone into smaller components which then can be captured within the associated basket.
The size, stiffness, and length of the straight shafts in much of the existing ultrasonic lithotripter technology only allow the use of such devices with large shafts in percutaneous procedures (i.e., direct access to stones in the kidney through a small incision in the patient's back and through the kidney itself). Percutaneous procedures are usually only performed in the United States for very large kidney stones, in lieu of addressing such stones via flexible scope procedures, which would require a very long duration to complete. Percutaneous procedures seem to be more frequently used in countries outside of the United States, possibly due to the high cost and usually fragile nature of the flexible ureteral scopes. There is some evidence that percutaneous procedures are even used for smaller stones outside of the United States, possibly due to cost and fragility of, and risk to flexible ureteroscopes.
Laser lithotripsy is a strong competitor of ultrasonic lithotripsy. Laser energy passing through the laser fibers can be used to very effectively break the kidney stones in virtually any area of the urinary system. When used with flexible ureteroscopes, laser fibers can bend around corners and access kidney stones in the lower pole of the kidney. Perhaps since lasers have been known to break in the working channel and damage flexible ureteroscopes, techniques have been developed to access and retrieve kidney stones in the lower pole of the kidney and move them to a different location such as the upper pole of the kidney where they are more accessible.
Electrohydraulic lithotripsy (EHL) has similar ease and access via flexible endoscope to laser lithotripsy with generally lower cost, but with also generally lower stone fragmentation efficiency. When using this technology, there are also some concerns about local shockwave effects of nearby tissue.
Most, if not all, current ultrasonically or pneumatically driven lithotripsy shafts are distally terminated to be smooth and perpendicular to the shaft axis. This smooth, flat surface, while providing more protection to soft tissue because of its smoothness, can make it extremely easy for the activated shaft to slip off the stone, or for the stone to slide out from beneath the vibrating smooth tip. This may prolong duration of a stone breaking procedure because the physician must “chase” the stone around to break it up.
A common design configuration for an ultrasonic lithotripsy drive component tends to be a stack assembly of piezoelectric discs, such as 4 to 6 in number, with an approximately 15 to 20 millimeter outside diameter, a length of approximately 20 to 30 millimeters, and an inner diameter of approximately 7 to 10 millimeters. Each piezoelectric disk in such a stack assembly can have a thickness of about 3 to 4 millimeters. The stack configuration provides for multiplication of the dimensional changes each piezoelectric disk undergoes with various voltage levels and polarities are applied across the body of each disk.
The thickness of each disk is part of what determines the voltage that must be applied to achieve a specific dimensional change. For example, if one disk expands longitudinally by 1 μm (micrometer) from application of certain voltage potential at the two main faces of the respective disk, a stack of 6 such disks, with each disk subjected to the same voltage potential applied across it should expand by 6 μm. The longitudinal expansion of the disk can be further increased by the utilization of a focusing cone configuration, which then transfers and magnifies the disks longitudinal expansion to drive a lithotripsy shaft forward and backwards and/or excite longitudinal vibration energy in the shaft.
Such configurations, especially with individual piezoelectric disks with a thickness of 3-4 millimeters, require either significantly high voltages to induce significant dimensional changes, or are highly dependent on operating at a specific resonant frequency to be effective when using drive voltages within a practical range. Other components coupled to such drivers must conform to particular resonant frequency requirement in order to be effective with such a drive that has a resonance dependence for effective operation.
Thinner piezoelectric discs are much more responsive to voltage stimulus than are thicker discs. Stack assemblies with thinner piezoelectric disks are less dependent on a specific resonant frequency to be effective at a longitudinal dimension change, but to achieve the same level of overall longitudinal dimension change, many more elements are needed (to essentially achieve the same total thickness of a thick disk stack). Thus, complexity and price of a respective driver rise considerably when using thinner disks.